This functionality is now live!
You will be able to access these reports in the Service Hub Analytics Suite (Reporting > Service Analytics > Ticket replies) or create these reports from scratch in the Custom Report Builder (Reporting > Reports > Create new report > Custom Report Builder) in the section “Service” and “Help Desk messages”.
You can also view 2 Message Analytics out-of-the-box reports in the Help Desk Analyze Tab (Workspace > Help Desk > Analyze).